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From thescribe's parallel comment it sounds like you're describing doctests which is a built in feature in Elixir, and I believe in Python also. Many languages have doctest libraries available, though not many support it being part of canonical documentation... I've been meaning to try the ruby-rspec doctest library https://github.com/doctest/doctest-rspec though there's several others besides that one

Rust. https://doc.rust-lang.org/book/second-edition/ch11-03-test-o...

https://dlang.org/spec/unittest.html for D

I don't think it is an official part of the language but the Pragmatic Programmers Elixir book does this in one of their examples code sections.

It is an official part of the language, and is automatically part of the compiled docs, which are also an official part of the language! https://elixir-lang.org/getting-started/mix-otp/docs-tests-a...